Characterization of the isothermal solidification process in the Ni/Au-Ge layer system

The eutectic Au28Ge (at.%) solder alloy in combination with Ni substrates or Ni coatings is interesting for applications in transient liquid phase (TLP) bonding. To achieve sound joints with high re-melting temperatures, it is necessary to gain profound knowledge about the joining process. In this work, the isothermal solidification behavior in the Ni-Au28Ge (at.%) system was analyzed by means of microstructure evolution and time to complete solidification. In short-time annealing experiments the early stage of the reaction (< 300 s over liquidus temperature) was investigated. It was observed that a continuous layer of Ni5Ge3 forms at the substrate-solder interface before scallop-like NiGe grains start to grow as solidification proceeds. Cyclic differential scanning calorimetry measurements were applied to estimate the time to complete solidification. Solidification was completed after less than 1 h in the Ni-Au28Ge (at.%) system, which enables comparably short processing times for TLP bonding.